You can fill the third textbox with the second checkbox click itself like below...
private void checkBoxMonthly_CheckedChanged(object sender, EventArgs e)
{
int grandTotal;
SQLHelper objsql = new SQLHelper();
objsql.SqlText = " select SettingID,SettingValue from tblSettings where SettingID = 2";
objsql.AddParameter("FeetypeID", checkBoxMonthly.Checked);
DataTable dt = objsql.getDataTable(false);
if (dt.Rows.Count > 0)
txtMonthlyFee.Text = dt.Rows[0]["SettingValue"].ToString();
objsql.Close();
objsql.Dispose();
grandTotal = Convert.ToInt32(txtAdmissionFee.Text) + Convert.ToInt32(txtMonthlyFee.Text);
txtGrandTotal.Text = grandTotal.ToString();
}
NOTE: Take datatype and conversion mechanism as per your requirements.
Thanks...